1. Field of the Invention
This invention relates to an antifuse element provided on a semiconductor device which is used for, for example, a field programmable gate array (FPGA) or programmable read only memory (PROM) constituted as an integrated circuit.
2. Description of the Related Art
A semiconductor device used for a FPGA (which is a gate array programmable by a user) or PROM is generally provided with antifuse elements which comprise a bottom electrode, an antifuse material layer, and a top layer electrode, as is described in IEEE, Electron Device Letter, Vol. 12, No. 4, April 1991 pp. 151-153, and IEEE, IEDM Tech.Dig 1993 pp. 31-34.
In such an antifuse element, an antifuse material insulating film is formed within a through hole between the bottom and top electrodes, and at a selected antifuse element, the antifuse material film is broken down to form a filament for electrical connection between the bottom and top electrodes. In order to break the antifuse material film, a relatively high breakdown voltage is applied to the selected antifuse element between the bottom and top electrodes. By employing this type of antifuse element, desired programming is easily achieved by a user for FPGA because antifuse elements to be connected are arbitrarily selected after forming metal interconnects to provide conductivity between the bottom and top electrodes. Similarly, desired data writing is allowed for PROM after forming the metal interconnects.
However, there are still some problems in such conventional antifuse elements.
First, the bottom electrode of the conventional antifuse element described in the above publication is made of TiN which also functions as a barrier metal. The TiN has a column crystalline structure and the resultant electrode has sharp protrusions on its surface. That is, the surface of the conventional bottom electrode is rough and uneven. When forming a very thin antifuse material film having a thickness of several ten nm on the rugged surface, the electric field for breaking down the antifuse material film is concentrated to the protrudent portions, and the breakdown condition of the antifuse material film differs among the antifuse elements or differs among the area even in the same through hole. As a result, variations are caused in the cross sectional area of the filaments which are formed by the breakage of the antifuse material film, which further causes variations in ON resistance when the operation voltage is applied between the bottom and top electrode, and results in variations in the extent of the delay of the interconnect wiring. Consequently, the program reading speed for RPGA and the data reading speed for PROM are also varied. More particularly, the filament formed on the surface having sharp protrusions has a smaller cross section, and naturally, the 0N resistance during the application of the operation voltage becomes high. Moreover, the filament in the conventional antifuse element is formed by a compound containing not only titanium (Ti) but also nitrogen (N) from the bottom electrode as well as sillicon (Si) from the insulating film, and the resistance of the filament itself becomes high. Such high resistance causes a large delay in the wiring, and consequently, the reading speed of the program or data is reduced.
Second, electrical connection must be ensured between the top and bottom electrode when the antifuse material film is broken down, while insulation must be ensured where no breakdown occurs. For this reason, in the conventional antifuse elements, it was necessary to make the antifuse material film thicker, including margin thickness for reliable insulation, due to the large surface roughness of the bottom electrode. This causes the breakdown voltage to be raised. However, in view of the fact that an integrated circuit becomes more and more dense, the operation voltage of the IC must be reduced. Therefore, in order to provide a high breakdown voltage for break downing the antifuse material film, separate means, such as a booster, is required.